1. Field of the Invention
The present invention relates to wireless network communications. More particularly, the present invention relates to a Time Division Multiple Access (TDMA) system and method.
2. The Background Art
Time Division Multiple Access (TDMA) is well known in the art as a method of utilizing bandwidth across a shared medium. The basic principle of time division multiplexing is that it permits a user to have access to a TDMA system which operates at a rate which is several times greater that the rate required to support the user's own data throughput.
Several common wireless protocols employ TDMA, such as the Global Mobile System (GSM), Digital Enhanced Cordless Telecommunications (DECT), and Bluetooth. GSM is a European standard which employs simultaneous TDMA and Frequency Division Multiple Access (FDMA) schemes in which each frequency used for communication is divided into TDMA slots. With GSM each TDMA device is assigned slots for transmit, receive and base station communications. All GSM communications in the system take place using the Gaussian Minimum Shift Keying (GMSK) modulation technique.
DECT is a standard developed by the European Telecommunication Standard Institute for use in European digital mobile telephone systems. DECT uses frames of 24 slots allowing up to 12 channels for transmit and 12 channels for receive. Each device on the DECT network uses the same modulation technique, but the system allocates varying bandwidth to devices by assigning multiple slots or through the use of padding to stretch packets to uniform lengths.
Bluetooth is a technology specification for small form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable devices. Additionally, Bluetooth is an emerging standard for consumer wireless devices which uses TDMA. Under this standard, devices can be assigned one or more 625 μs time slots in which to transmit or receive. Each device uses the same modulation technique. Different bandwidth devices are accommodated by assigning multiple contiguous timeslots to devices with increased throughput requirements.
For each of the wireless protocols described above, a common modulation scheme is employed by all devices on the network. Additionally, a common wireless medium is allocated to devices with different bandwidth requirements to share the same network. Furthermore, in each of the previously described protocols, increased bandwidth demands for high bit-rate devices are satisfied by assigning longer time slots to the high bandwidth device. Low bit-rate devices are accommodated by allowing them to zero-fill assigned slots up to the minimum slot length supported.
Therefore, it would be beneficial to allow devices with different bit-rates to communicate with one another. Previous systems have been employed where a TDMA scheme allows different devices to be assigned different time slots at different bit-rates.
U.S. Pat. No. 4,201,892 ('892) titled “Multi-Rate TDMA Communication System” is one system developed for satellite communications in which two different bit-rates are supported in the same TDMA frame using two different frame reference bursts. At the beginning of each frame, the primary frame reference burst is transmitted followed by one or more time-slots assigned to devices capable of communicating using the primary bit-rate. After the primary bit-rate slots, a device synchronized to the primary frame reference burst transmits a secondary frame reference burst at a secondary bit-rate. Devices capable of transmitting at the secondary bit-rate are then allowed to transmit slots following the secondary frame reference burst. The system is configured so that the secondary slots are completed before the next primary frame reference which begins the following frame.
This system and method of interleaving two or more classes of different bit-rate devices has limited application for a variety of reasons. For example, the system requires multiple frame reference bursts and divides the frame into separate sections isolated by these frame reference bursts. It would therefore be beneficial to provide a system that does not require multiple frame reference bursts and allows devices of varying bit-rates to be assigned slots at any point within the frame. By allowing devices to be assigned slots anywhere within the frame, there is greater flexibility when devices of different bandwidths are continuously signing one and off from the network.
U.S. Pat. No. 4,586,177 ('177) titled “Integrated Narrowband and Wideband TDMA Networks” is another teaching which allows different devices to be assigned different time slots at different bit-rates. Like the '892 patent, the network system provides that different reference bursts are used to divide the frame into sub-frames supporting devices of differing bit rates. However, unlike the '892 patent, the system disclosed in the '177 patent divides the frame to support two types of devices. The system supports wideband devices which are assigned the complete bandwidth during TDMA slots as well as narrowband devices which share the bandwidth during TDMA slots using a FDMA technique.
However, the network system disclosed in the '177 patent is limited to the use of multiple frame reference bursts. Additionally, the patent is limited to devices having specific bit-rates beings assigned slots following their respective reference bursts. Further still, each of these bursts are transmitted at a different carrier frequencies.
Another system in the prior art is described in “A Time Division Multiple Access System for the Defense Satellite Communication System” by Husted and Walker, appearing in the reports of the 1970 EASCON, pages 229–237. This system uses an external timing source for framing information and does not use a frame reference burst.
The limitation of Husted et al.'s TDMA Sattellite Communication System is that any device participating in this system must be capable of demodulating the highest bit rate signal. Therefore, the low bit-rate devices must include all electronics and timing necessary to receive at the higher bit-rate and prevents low-cost, low bit-rate devices from being developed.
Therefore, it would be beneficial to provide a network which would allow low bit rate devices to communicate with high bit rate devices.
It would also be beneficial to provide a system and method for devices operating with different modulation methods to communicate with one another.
Further still it would be beneficial to provide a system and method wherein a master device synchronizes the communications between slave devices, and the communications may be accomplished by varying the pulse repetition frequency, the modulation technique, the TDMA frame slot size, and the number of slots in a TDMA frame.